This invention relates to coated particles, particularly particles with an inorganic content, preferably purely inorganic particles and most preferably coated magnetic particles, which are used as carrier particles in electrostatographic developers for the electrostatographic production of images.
There is a series of electrostatographic printing methods which are known, e.g. direct electrostatic printing, in which toner is deposited by means of an electronically addressable print head on to a receiver material which does not comprise a latent electrostatic image.
In another form of electrostatic printing, toner formers are produced on an image-producing element in the form of a rotating drum which contains an electrostatic layer which consists of a multiplicity of controllable electrodes in and under a dielectric layer. An electrical voltage is generated in the controllable electrodes, according to the image, and attracts toner particles from a toner source.
In addition, it is known that in electrographic printing and in electrophotographic copying a latent electrostatic image can be produced, either of an original to be copied or for digitised data which describe an image which is accessible electronically.
In electrophotography, a latent electrostatic image is produced by the steps of a) charging a photoconductive element congruently, and b) discharging according to the image by an exposure which is modulated image by image.
In electrographics, a latent electrostatic image is produced by the deposition of electrically charged particles corresponding to the image, e.g. on a dielectric substrate by an electron beam or by ionised gas.
The latent image formers obtained are developed, namely they are converted to visible images by depositing substances on them which selectively absorb light; these substances are called toners and usually carry a triboelectric charge.
Two techniques are used for the toner development of latent electrostatic images: dry powder and liquid dispersion development, with dry powder development being the more commonly used.
Dry powder development can be carried out in various ways. One method is the single component method, in which the toner itself is charged by friction, transported by a roller and deposited on the latent image. The quality is limited, particularly if coloured prints are to be produced. Another method utilises liquid development, in which colloidally charged toner particles are applied to the photoconductor in a liquid insulating medium, e.g. a hydrocarbon. One disadvantage of this method is the emission of volatilised organic substances, particularly when high printing speeds are employed. A third method utilises a two-component developer. In this case, large-grained carrier particles which can be attracted magnetically form a magnetic brush on the surface of the developer roll, by forming the magnetic hairs of the brush. Triboelectrically charged toner particles are present on the surface of the carrier particles. These are stripped from the carrier particles according to the electrical charge of the latent electrostatic image, whereby the toner image is produced. In this method, the carrier particles are reused repeatedly; their mechanical stability is therefore particularly important.
Non-magnetic electrophotographic developers comprising two components can also be used as an alternative to what is termed the magnetic brush. In one particular embodiment, the developer consists of small glass spheres, which arc optionally coated, and of toner particles.
The developer is caused to deposit on the element carrying the latent image and in this manner causes development, as is described in BE 828 210. The carrier particles are also reused in this case.
In the case of surface-coated carrier particles, the stability of the surface coating is particularly important. Insufficient mechanical stability results in losses of quality of the printed image, and the developer has to be replaced, which causes unnecessary stoppages and cost.
A material with anti-adhesion properties is preferred for coating the carrier particles, in order to prevent the toner from sticking to the carrier surface. In most cases, however, this leads to a decrease in the adhesion of the coating on the carrier, which results in a shortening of the service life, e.g. when silicone resins are used as carrier coating media.
Carriers are known from U.S. Pat. No. 4,977,054 which consist of a magnetic powder and a silicone resin coating. The silicone resin used consists of D and T structural units (in D units the silicon atom is linked to further silicon atoms via 2 oxygen atoms, in T units it is linked via 3 oxygen atoms), wherein other functional organosilanes, such as di- and trialkoxy-functional organosilanes and/or di- and trialkoxy-functional organosilanes which contain nitrogen, can also be added in addition. Deposition is effected, as can be inferred from the examples, in a fluidised bed reactor; thereafter, the coatings are also cured at 190xc2x0 C. to 296xc2x0 C.
However, the aforementioned coatings comprising (polymeric) silicone resins have the disadvantage that high temperatures (190 to 296xc2x0 C.) are necessary for complete curing. Furthermore, many silicone resin coatings in fact exhibit anti-adhesion properties, and inevitably exhibit poor bonding to the substrate. According to U.S. Pat. No. 4,4977,054, di- and trialkoxy-functional organosilanes, for example, therefore have to be added to these silicone resins in order to improve their adhesion.
The object of the present invention was therefore to provide coated particles, particularly particles having a content of inorganic materials, and particularly carrier particles for electrostatographic processes, wherein the particles have a magnetic core and a coating on the core which
a) is non-sticky, so that free-flowing particles which are predominantly free from agglomerates are obtained,
b) does not soften, even at elevated temperature,
c) prevents permanent adhesion and adhesive bonding of colour-imparting toners,
d) has a high resistance to abrasion,
e) adheres well to the particles, and
f) in the case of carrier particles for electrophotography has a good charging capacity, so that sufficient toner is taken up and can be released again.
The object of the present invention was also to provide a process for coating particles, particularly particles having a content of inorganic material, and particularly magnetic particles, which does not have the aforementioned disadvantages and which is characterised, for example, in that the deposition of the coating is effected in apparatuses which are as simple as possible by a simple process, and curing of the coating is effected at low temperatures.
Surprisingly, it has now been found that coated particles which do not have the aforementioned disadvantages are obtained, particularly coated particles containing inorganic material, if the coating is produced from monomeric polyfunctional organo-silanes and/or hydrolysis products thereof and/or reaction products thereof with organo-silanes containing hetero atoms and/or alkoxides.
The present invention therefore relates to particles (A) which are coated with a material (B), wherein particles (A) preferably contain inorganic material and are most preferably magnetic carrier particles for electrostatographic processes, and wherein material (B) is a monomeric, polyfunctional organosilane and/or a hydrolysis product thereof and/or a reaction product thereof with an organosilane containing a hetero atom and/or an alkoxide.
Organosilanes which contain hetero atoms in the sense of this invention consist of at least one silicon atom with hydrolysable and/or condensation crosslinking groups such as xe2x80x94SiOR, wherein R represents alkyl, cycloalkyl or aryl in particular, preferably alkyl, or consist of SiOH and at least one organic radical which contains a hetero atom and which is bonded via a carbon atom, which may be an alkyl, cycloalkyl or aryl radical. The hetero atoms of the organosilanes which contain hetero atoms are preferably N, P, S, F, Cl, Br, O, Band Al.
The preferred hetero atoms are N and F, wherein nitrogen atoms are particularly preferred.
The preferred organosilanes which contain nitrogen correspond to formula (I)
(R2)2xe2x80x94N[(CH2)mNR2]n(CH2)m Si(OR3)3xe2x88x92o(R4)oxe2x80x83xe2x80x83(I)
wherein
m represents 1 to 10, preferably 2 or 3,
n represents 0 to 2, preferably 2,
o represents 0 to 2, preferably 0,
R2 represents H, alkyl or aryl, preferably H, and
R3, R4 represent alkyl or aryl, preferably CH3 or C2H5.
The preferred alkoxides correspond to formula (II)
M1(OR1)yxe2x80x83xe2x80x83(II),
wherein
M1 represents Si, Sn, Ti, Zr, B or Al,
R1 represents alkyl or aryl, preferably a C1-C4 alkyl, and
y represents 4 in the case of Si, Sn, Ti and Zr, and represents 3 in the case of B or Al.
Polyfunctional organosilanes in the sense of the invention are characterised in that they contain at least 2, preferably at least 3 silicon atoms, each with 1 to 3 hydrolysable and/or condensation crosslinking groups, particularly alkoxy, acyloxy or hydroxy groups, and the silicon atoms are each bonded by a Sixe2x80x94C bond to a structural unit linked to the silicon atoms.
In the simplest case, examples of linked structural units which are suitable in the sense of the invention include linear or branched C1 to C10 alkylene chains, C5 to C10 cycloalkylene radicals, aromatic radicals, such as phenyl, naphthyl or biphenyl for example, and combinations of aromatic and aliphatic radicals also. The aliphatic and aromatic radicals may also contain hetero atoms, such as Si, N, O, S or F for example.
In addition, siloxanes in chain, ring or cage form, such as silsesquioxanes, are also suitable as linked structural units.
Examples of linking structural units are listed below, wherein X denotes Si atoms which have 1 to 3 hydrolysable and/or condensation crosslinking groups, and Y denotes corresponding Si atoms which are bonded via an alkylene chain to the linking structural unit; n represents a number from 1 to 10, and m represents a number from 1 to 6:
Xxe2x80x94(CH2)nxe2x80x94X;

wherein R is an organic radical, e.g. alkyl, cycloalkyl, aryl or alkenyl.
Examples of polyfunctional organosilanes include compounds of general formulae (IV), (V) and (VI):
xe2x80x83(R5)4xe2x88x92iSi[(CH2)pSi(OR6)a(R7)3xe2x88x92a]ixe2x80x83xe2x80x83(IV)
wherein
i represents 2 to 4, preferably 4,
p represents 1 to 4, preferably 2 to 4,
R5 represents alkyl or aryl,
R6 represents hydrogen, alkyl or aryl when a is 1 and alkyl or aryl when a is 2 or 3,
R7 represents alkyl or aryl, preferably methyl, and
a represents 1 to 3; 
wherein
m represents 3 to 6, preferably 3,
q represents 2 to 10, preferably 2,
b represents 1 to 3,
R8 represents a C1-C6 alkyl or a C6-C14 aryl, preferably methyl or ethyl,
R9 represents hydrogen, alkyl or aryl when b is 1, or alkyl or aryl when b is 2 or 3, and
R10 represents alkyl or aryl, preferably methyl;
(R14)4xe2x88x92kSi[OSi(R11)2(CH2)r Si(OR12)c(R13)3xe2x88x92c]kxe2x80x83xe2x80x83(VI)
wherein
r represents 1 to 10, preferably 2 to 4,
c represents 1 to 3,
k represents 2 to 4, preferably 4,
R11 represents alkyl or aryl, preferably methyl,
R12 represents H, alkyl or aryl, preferably H, CH3, C2H5 or C3H7, when c is 1; and alkyl or aryl, preferably CH3, C2H5 or C3H7, when c is 2 or 3,
R13 represents alkyl or aryl, preferably methyl, and
R14 represents alkyl or aryl.
Examples of polyfunctional organosilanes include:
(a-1) Si[(CH2)2Si(OH)(CH3)2]4,
(a-2) H3Cxe2x80x94Si[(CH2)2Si(OH)(CH3)2]3,
(a-3) C6H5xe2x80x94Si[(CH2)2Si(OH)(CH3)2]3,
(a-4) Si[(CH2)3Si(OH)(CH3)2]4,
(a-5) cyclo-{OSiCH3[(CH2)2Si(OH3)(CH3)2]}4,
(a-6) cyclo-{OSiCH3[(CH2)2Si(OCH3)(CH3)2]}4,
(a-7) cyclo-{OSiCH3[(CH2)2Si(OCH3)2CH3]}4,
(a-8) cyclo-{OSiCH3[(CH2)2Si(0C2H5)2CH3]}4,
(a-9) cyclo-{OSiCH3[(CH2)2Si(OC2H5)3]}4.
Examples of alkoxysilanes which contain nitrogen include:
(b-1) H2Nxe2x80x94(CH2)3Si(OCH3)3 
(b-2) H2Nxe2x80x94(CH2)3Si(OC2H5)3 
(b-3) H2Nxe2x80x94(CH2)2xe2x80x94HNxe2x80x94(CH2)3Si(OCH3)3 
(b-4) H2Nxe2x80x94(CH2)2xe2x80x94HNxe2x80x94(CH2)3Si(OCH3)2(CH3)
(b-5) C6H5xe2x80x94HNxe2x80x94(CH2)3Si(OCH3)3 
(b-6) C6H5xe2x80x94HNxe2x80x94(CH2)3Si(C2H5)3 
(b-7) H2Nxe2x80x94(CH2)2xe2x80x94HNxe2x80x94(CH2)2xe2x80x94HNxe2x80x94(CH2)3Si(OCH3)3 
(b-8) H2Nxe2x80x94(CH2)2xe2x80x94HNxe2x80x94(CH2)2xe2x80x94HNxe2x80x94(CH2)3Si(OC2H5)3.
Examples of alkoxysilanes which contain fluorine include:
(d-1) F3Cxe2x80x94(CH2)2xe2x80x94SiRxe2x80x23xe2x88x92x(OR)x 
(d-2) F3Cxe2x80x94(CF2)7xe2x80x94(CH2)2xe2x80x94SiRxe2x80x23xe2x88x92x(OR)x 
(d-3) (F3C)2CFxe2x80x94Oxe2x80x94(CH2)3xe2x80x94SiRxe2x80x23xe2x88x92x(OR)x 
(d-4) (3-F3Cxe2x80x94C6H4)xe2x80x94SiRxe2x80x23xe2x88x92x(OR)x 
(d-5) (3-F3Cxe2x80x94C6H4)2Si(OR)2.
wherein x is 1 to 3 and R and Rxe2x80x2 are alkyl, cycloalkyl or aryl, preferably ethyl or methyl.
Examples of alkoxides which can be used for the production of the reaction products according to the invention, e.g. in order to obtain an improvement in abrasion-resistance or in the tribological properties, include:
(c-1) Si(OC2H5)4 
(c-2) B(OC2H5)3 
(c-3) Al(Oxe2x80x94ixe2x80x94C3H7)3 
(c-4) Zr(Oxe2x80x94ixe2x80x94C31H7)4.
In addition, the reaction products according to the invention can also contain finely divided metal oxides or metal oxide-hydroxides of the elements Si, Sn, In, Tl, Zr, B or Al, e.g. silica sols, which contain organic solvents in particular. The preferred primary particle size thereof falls within the range from 1 to 50 nm; they are hereinafter termed xe2x80x9cnanoparticlesxe2x80x9d. Agents which induce conductivity, e.g. carbon and charge-regulating agents e.g. nigrosine, can also be added to the coating.
Material B preferably contains 0.1 to 100% by weight of polyfunctional organosilane, 0 to 20% by weight of organosilane (I) which contains a hetero atom, 0 to 70% by weight of nanoparticles and 0 to 99.9% by weight of alkoxide (II). Material B most preferably contains 20 to 80% by weight of polyfunctional organosilane, 20 to 80% by weight of alkoxide (II), 0 to 10% by weight of organosilane (I) which contains a hetero atom, and 0 to 50% by weight of nanoparticles.
Magnetic inorganic particles are preferred as the particles.
The magnetic particles are preferably iron oxide pigments of formula (III)
(MO)x(Fe2O3)2xe2x80x83xe2x80x83(III)
wherein
M is Li, Mg, Sr, Ba, Mn, Fe(II), Co, Ni, Cu, Zn or Cd, and the molar ratio of x to z is between 0 and 1, preferably between 0.3 and 1.
It is also possible to use composite particles which consist of 20 to 85% by weight of magnetic microparticles and an organic or inorganic binder, e.g. an organic polymer or a ceramic material. Moreover, it is possible to use non-magnetic cores such as glass beads.
Reaction products B) according to the invention are generally deposited on particles A) as a coating. The polyfunctional organosilanes can be deposited on particles A in solvent-free form or dissolved in a solvent, optionally in the presence of a catalyst. Coating B) is obtained after volatilisation of the solvent and curing at a suitable temperature.
In one preferred embodiment, the polyfunctional organosilanes are first mixed, optionally in a solvent, with alkoxides and with organosilanes containing hetero atoms, for example, and are deposited on particles A, optionally in the presence of a catalyst, and cured. In order to increase the reactivity and to reduce the drag-out of low-boiling, readily volatile starting materials such as tetraethyl orthosilicate, it is particularly advantageous firstly to react the starting materials with water, optionally in the presence of a catalyst. Reactive, less volatile, oligomers and/or polymers are formed in the course of this procedure.
After a defined reaction time, this coating solution is deposited by a suitable method, e.g. in a fluidised bed, on the materials which contain iron oxide, the volatile constituents are volatilised and the coating which is thus obtained is optionally subsequently cured thermally.
Suitable catalysts include organic and inorganic acids or bases, e.g. HCO2H, CH3COOH, HCl, NH4OH and alkali metal hydroxides, as well as salts containing F such as NaF or NH4F. The added metal oxides themselves, such as Ti(OC2H5)4 and Ti(Oixe2x80x94C3H7)4, may also have a catalytic effect. Metal soaps such as zinc octoate or dibutyltin laurate may also be used.
Information on the hydrolysis and condensation of polyfunctional organosilanes, optionally in the presence of alkoxides, is to be found, for example, in DE-OS 196 03 242 and in WO 94/06897. For example, the polyfunctional organosilanes are mixed with the alkoxides, a solvent, water and a catalyst with stirring and are allowed to react for a defined period before films or, after complete reaction (gelling), even shaped bodies can be obtained from these solutions. The film-forming properties of reaction product (B) constitute a useable indication that the solutions are suitable for coating the particles. If a glass plate is coated, for example, a transparent, substantially fissure-free film which wets the entire area should be obtained after volatilisation of the volatile constituents. The tendency to form fissures increases with the layer thickness of the film, however.
In one preferred embodiment of the process according to the invention, the alkoxysilanes which contain nitrogen are not added until the polyfunctional organosilanes and optionally the alkoxides have been reacted as described above and this preliminary condensate has been diluted with further solvent. As is known from the literature, silanes which contain nitrogen, such as H2Nxe2x80x94(CH2)3Si(OMe)3, catalyse the hydrolysis and condensation of alkoxysilanes. This can result in the most reactive component, e.g. H2Nxe2x80x94(CH2)3Si(OCH3)3, being rapidly hydrolysed and being condensed to form an insoluble solid. This can be prevented by adding the alkoxysilanes which contain nitrogen to the diluted coating solution.
In order to obtain a homogeneous distribution of the coating solution on all the particles which contain iron oxide, it is advantageous to dilute the preliminary condensate further with additional solvent. Moreover, the further reaction of the polyfunctional organosilanes and alkoxides which are used proceeds considerably more slowly in dilute solutions, which results in improved stability of the coating solutions on storage.
Examples of suitable solvents which can be used for the dilution of the preliminary condensate include alcohols such as methanol, ethanol, n-propanol, n-butanol, iso-propanol, sec-butanol or ethylene glycol, ketones such as acetone or methyl ethyl ketone, amides such as N-methylpyrrolidone, or even water. Alcohols, particularly iso-propanol, are preferred, due to their good miscibility with the preliminary condensate. Mixtures of different solvents may also be used.
After the deposition of the coating solution, the solvents can be recovered, e.g. by condensation, and can be re-used in the process, optionally after purification.
Curing of the coating is preferably effected at temperatures from 25 to 220xc2x0 C., more preferably from 80 to 180xc2x0 C., most preferably from 100 to 140xc2x0 C.
The amount of coating deposited with respect to the core is between 0.1 and 10% by weight, preferably between 0.5 and 5% by weight, and most preferably between 0.5 and 2% by weight.
In particular, the coated inorganic particles, particularly carrier particles for electro-photography which contain a magnetic core, are spherical and have an average particle diameter of 20 to 200 xcexcm, preferably 40 to 120 xcexcm.
It is also possible to deposit two or more layers on the particles (A), e.g. firstly an electrically insulating layer and above this a layer which improves the stability of the coated particles under mechanical stress.